Dissecting the Determinants of cAMP Affinity in Protein Kinase A

Abstract

cAMP receptors contain highly conserved cAMP binding pockets, in part responsible for allosteric activation, yet CBDs exhibit a wide array of cAMP binding affinities. While several cAMP:CBD crystallographic structures have been solved, they are insufficient to explain differences in cAMP:CBD affinities. We hypothesize that it is the position of the apo autoinhibitory equilibrium and/or a change in the state-specific association constants of the active and inactive CBD forms that are primarily responsible for modulating ~1000-fold difference in cAMP affinities. Interestingly, we discovered that PKARIα and HCN2 have comparable state-specific association constants, suggesting that the position of the apo autoinhibitory equilibrium is primarily responsible for the large difference in observed cAMP affinities in these systems. In addition, the individual components of the cAMP binding pocket (i.e. BBR, PBC, and lid) show functional variability across different CBDs. In RIα, both the BBR and lid are dispensable for high affinity cAMP binding, leaving the PBC as the key determinant of cAMP affinity. Interestingly, in addition the PBC:cAMP contact side-chains, non-contact side-chains are also important in modulating cAMP affinity (ie. L201 and Y205). Further dissection of the contributions arising from the apo pre-equilibrium and the cAMP binding pockets is required to better understand cAMP affinity and selectivity.